a) Field of the Invention
The present invention relates to a lead frame and the manufacture of semiconductor devices, and more particularly to a method of manufacture a lead frame and a semiconductor device capable of improving process yield.
b) Description of the Related Art
Lead frames used for the assembly of LSI devices such as shown in FIGS. 9 and 10 have been proposed heretofore.
A lead frame 10 is formed by punching a lead frame material plate made of, for example, alloy 42 (Fe alloy containing 42% Ni). A support member (die stage) 12 for supporting a semiconductor chip is suspended from an outer frame 11 by tie bars (suspension pins) 14a to 14d. A number of leads 16 supported by the outer frame 11 and dam bars 15 are disposed around the support member 12, front tips of the leads 16 being spaced apart from the support member 12.
A lead frame 10 is generally of a long strip of a lead frame material plate, and a plurality of (e.g., five) assembly units each having the support member 12 and leads 16 such as shown in FIG. 9 are placed side by side. Near at the side edges (upper and lower edges as viewed in FIG. 9) of the lead frame along its longitudinal direction, positioning holes 10a and 10b are formed for positioning the lead frame when it is mounted on a jig. As shown in FIG. 10, each lead 16 has an inner lead 16x and an outer lead 16y. The inner lead is a portion of the lead which is covered with molding resin, and the outer lead is a portion of the lead which is exposed at the outer area of the molding resin region. The dam bar 15 coupling together the outer leads is removed through punching or the like after the molding process.
With reference to FIGS. 11 to 13, an example of conventional assembly methods will be described. At the process illustrated in FIG. 11, an LSI chip 18 is adhered to the support member 12 with Ag paste or the like. In this case, bonding pads formed on the LSI chip 18 are on the top surface opposite to the adhesion surface. The adhesion state of the LSI chip is also shown in FIGS. 9 and 10. Thereafter, a plurality of bonding pads on the LSI chip 18 are connected by bonding wires 20A and 20B to corresponding inner leads of the leads 16A and 16B which are illustratively shown among a number of leads 16.
At the process illustrated in FIG. 12, the chip 18, bonding wires 20A and 20B, and inner leads of the leads 16A and 16B are hermetically sealed with resin 22. A broken line 22S shown in FIG. 10 indicates a contour of the resin 22.
At the next process illustrated in FIG. 13, the outer lead of each lead is solder plated when necessary, and thereafter each outer lead is cut to separate the respective leads and the assembly unit covered with the resin 22 from the lead frame 10. After the outer leads are cut, the leads 16A and 16B are shaped as shown in FIG. 13 which is a side view of an LSI device formed after the lead plating process, lead cutting process, lead shaping process, and the like.
In another conventional method already proposed, a lead frame in the form of a reel of tape is used for assembling discrete components such as transistors, the processes of chip adhesion, wire bonding, and resin molding are collectively performed, and thereafter the processes of lead plating, lead cutting, and lead shaping are sequentially and separately performed.
With the conventional technique described with FIGS. 9 to 13, each lead is supported, as shown in FIG. 10, only at its outer lead to the dam bar and outer frame of the lead frame, and the inner lead is not fixed positively except that it is connected to the outer lead. If an external force is applied, for example, to the front tip of the inner lead 16x while the lead frame is transported from one process to another, the inner lead becomes likely to bend or curve. This deformation of the inner lead 16x may often result in bonding defects of the bonding wires 20A and 20B.
As the mold 22 flows in the cavity of a molding machine during the resin molding process after wire bonding, the wires 20A and 20B become likely to be displaced from proper positions (wire sweep-away). This wire sweep-away may cause broken wires and/or wire short.
The assembly processes such as chip adhesion, wire bonding, and resin molding are performed separately. It is therefore necessary to transport a lead frame from one process to another, which lowers production efficiency. As described above, in order to improve the production efficiency, a continuous assembly method has been proposed in which a lead frame in the form of a reel of tape is used to collectively perform a plurality of assembly processes.
However, a lead frame in the form of a reel of tape is required to be wound around a reel. Therefore, the inner lead 16x is likely to be deformed and bonding defects may possibly occur. This continuous assembly method cannot be adopted therefore unless some countermeasure is taken into consideration.